Speedometer off?

[GasSavers_veloman]

I've got a good reason to believe my speedometer is reading about 10% high. What's the most accurate way to verify my speed? All I really can do is compare it to other cars, driving the same route. But who is to say those cars are right?

I drive a 97 civic, and have new tires on it, but they are the correct size. My friend driving his 07 Impala said we were only going 64mph when he was directly behind me on the highway, where my speedo said 70mph.
Every time I pass a police radar, it shows about 10% lower speed that my speedometer too. but then I look at mileage on yahoo maps for a certain trip and both match up well.

[1993CivicVX]

You could try mile markers on the highway. You could have your friend get an odometer reading on a 10 mile route and then see how many miles the route is for you. But your friend's might not be perfectly accurate either.

Type into Mapquest and Google as well as yahoo maps that certain trip you had and compare the distance of all three. Are they all the same?

[cfg83]

veloman -

Don't some GPS come with speed functions? Mooch one of those from a friend and compare in real time.

CarloSW2

[VetteOwner]

my hand held gps comes with speed and distance and estmated time of arrival etc all that fin stuff.

thats the best way to measure.

but yes do the mile marker thing. keep at a constant 60mph, have someone ride with you, have them have a stop watch and write down how long it takes to go 1 mile and keep repeating that. we did this and calculated my trucks to be 2 mph slower than it really is. and when i used my gps it said the same. the speedo was reading 2 mph low.

[QDM]

I agree with the other guys, a GPS is the way to go. Even the cheapest ones have a very accurate speed and odometer function. If your speedometer is off, chances are your odometer is off by the same 10%. Unfortunately you may be getting 10% worse gas mileage than you think!

My Swift is off by 5% with it's slightly larger than stock tires. Motorcycle is right on. Truck is 3% off in the other direction.

Q

[2TonJellyBean]

Time between the mile markers on a freeway at a steady speed multiple times if you can't access a GPS. They tend to be quite accurately spaced whenever I've used them.

MPH Min Sec
75 0 48.00
74 0 48.65
73 0 49.32
72 0 50.00
71 0 50.70
70 0 51.43
69 0 52.17
68 0 52.94
67 0 53.73
66 0 54.55
65 0 55.38
64 0 56.25
63 0 57.14
62 0 58.06
61 0 59.02
60 1 00.00
59 1 01.02
58 1 02.07
57 1 03.16
56 1 04.29
55 1 05.45
54 1 06.67
53 1 07.92
52 1 09.23
51 1 10.59
50 1 12.00
49 1 13.47
48 1 15.00
47 1 16.60
46 1 18.26
45 1 20.00

[GeekGuyAndy]

I've checked mine with GPS and anemometer (wind speed measuring device held out the window ) and they both agreed with the speedometer right on.

[cems70]

Originally Posted by QDM

If your speedometer is off, chances are your odometer is off by the same 10%. Unfortunately you may be getting 10% worse gas mileage than you think! Q

Not necessarily. My speedo is perfectly accurate, but my odometer is registering 4% too few miles.....according to my GPS. Before I swapped a Civic CX tranny into my Civic DX, both the speedo AND odometer were off, but by different percentages.

[Bill in Houston]

timing on mile markers is the way to go. your odo may be right, but the speedo could be wrong. of course if you can borrow a gps, that is the easiest and most accurate way to do it.

[GasSavers_bobski]

Originally Posted by cems70

Not necessarily. My speedo is perfectly accurate, but my odometer is registering 4% too few miles.....according to my GPS.

X2, but the other way around. With my current gauge cluster and VSS, the odometer is off by around 0.3% according to highway mile markers, and the speedo reads about 4% fast according to GPS readings.